This invention relates to optical instruments. More particularly, the invention is directed to devices for controlling and adjusting telescopes in the photography of astronomical objects.
Astronomical photography ordinarily requires long exposure times because of the low intensity of the light received at the earth from very distant objects. During the exposure period it is necessary to adjust the telescope and camera constantly to maintain the track on the object, which appears to move across the sky because of the motion of the earth. Failure to maintain this adjustment causes the image of the object to become elongated, a phenomenon known as trailing. The apparent motion of astronomical objects is magnified by a telescope, and a photograph of a star or galaxy through a telescope will simply be a streak across the film unless the telescope is adjusted constantly during the exposure to hold the image steady by tracking this apparent motion.
A conventional method for accomplishing this tracking is to insert into the optical system a device having an ocular with cross-hairs, through which a portion of the image field may be viewed. Such a device is termed a guider. One selects a particular star in this part of the image field and adjusts the device to center the cross-hairs on the star, called a guide star. As the camera exposure proceeds, one constantly varies the position of the telescope to keep the cross-hairs centered on this guide star. In this manner the image field is held stationary relative to the camera.
It is desirable to provide a guider which will not degrade or distort the main optical image in the camera. This is achieved by sampling only a small portion of the optical field off the optical axis. However, if this portion is too small the guide stars in the field become limited to the brightest stars, and the photographic objects available are correspondingly restricted. For the same reason, it is desirable to provide a guider which features the maximum adjustability to allow the greatest possible range of choices for selecting a guide star.
Conventional guiders include devices which are installed in front of the camera and sample a portion of the optical field by means of a mirror or prism off the optical axis. The sampled light is directed into an ocular having cross-hairs. Adjustability is achieved by rotation of the entire device around the optical axis. Unfortunately, presently available off-axis guiders consume an additional 2-3 inches of focal length of the telescope. This is a severe limitation in Newtonian telescopes, since the focal length is limited by the size of the secondary mirror. In the smaller telescopes of this type there is not sufficient focal length available for such off-axis guiders; the focal plane must lie within a short distance from the surface of the main telescope tube. Installation of an off-axis guider of the conventional type requires cutting a hole in the main tube and modification of the optical system, an undertaking of considerable effort and expense.
A further limitation of these conventional guiders is that the guide star is required to lie in a narrow annular band around the perimeter of the image field. Thus, if the object is centered in this field, one cannot track the telescope on guide stars that are too close to the object in the image plane. One can direct the light from other stars into the eyepiece by tilting the mirror or prism, but then the focal plane will no longer be perpendicular to the axis. This is a problem in Cassegrainian and Newtonian telescopes, and results in image distortion. Radial selection of guide stars in the optical field can also be provided by moving the mirror further into or out of the field. However this adjustment results in loss of available light to either the camera or eyepiece, an undesirable effect in either case. In short, there is no guider presently available that provides for radial selection of guide stars in a satisfactory manner.